Physiological AC Preamplifier

Introduction

The goal of this project was to build a small, cheap, light-weight preamplifier for student use in electrophysiology labs. Specifically:

• The cost should be less than $50.
• Gain should be 100-1000, a.c. coupled.
• Noise should be comparable to commercial units, but could be a little higher.
• Common-mode rejection must be very good so that the circuit will perform in environments with lots of 60 Hz noise.
• There should be a fixed 5kHz lowpass filter.
• There should be an optional 60 Hz notch filter.

The Circuit

The circuit is a "plain vanilla" preamplifier.

The input stage is the most critical for common-mode rejection and noise. We used a Burr-Brown INA121 differential amplifier, set to a gain of 10. The input stage is DC coupled. With a 9-volt supply, a differential DC offset of up to about a volt is tolerable. Offsets of a few hundred millivolts are typical, assuming similar metals for the differential electrodes. The common-mode rejection of the circuit is as good as the Burr-Brown head-stage.

Any one of the opamps in the circuit can be removed (along with its supporting resistors and capacitors) if the function is not needed. For instance, if the 60 Hz notch filter is not needed, output may be taken from the output of the third op-amp.

All four op-amps are on in one quad package.

Internal photo details:

A preprint of a paper describing this circuit and how we tested it is available. See also the crawdad download page for more details.